Flywheels are known for the storage of energy in the form of kinetic energy, for example for use in vehicles. In such instances it is known to use a flywheel to store the energy which would otherwise be converted to heat in the vehicle's braking system when the vehicle decelerates, this stored energy then being available for use to accelerate the vehicle when desired.
An existing type of flywheel according to FIG. 1 has a central metallic support section (1) which can be mounted on a central support such as a shaft. At least one composite ring (2) is mounted on the central support section. The composite ring in this type of flywheel is filament wound from carbon fibre. When the flywheel is in rotation, the ring will tend to expand in diameter due to the centrifugal forces acting on it. The ring has high strength in hoop for re-acting the centrifugal forces when the flywheel is in rotation. However, the outer ring can become a loose fit on the central support section and potentially (dangerously) become dismounted from the central support section. In addition the radial stress can result in failure of the composite ring.
In order to counteract the tendency of the ring to grow, the ring is typically machined with a smaller inner diameter than the outer diameter of the central support section and is then mounted onto the central support section with an interference fit. The mismatch in diameters results in a pre-load such that that ring exerts an inward force onto the central support section. This inward preload is greatest when the flywheel is not rotating and results in a requirement for the central support section to be sufficiently structurally strong that it can withstand the preload force when the flywheel is stationary. It is known for more than one composite ring to be pressed together and further mounted onto the central support. The pre-load increases towards the centre of the flywheel and with the number of rings pressed together. Consequently a large amount of material may be required in the central support section of the flywheel in order to counteract this pre-load force, and this material, being near the centre of the flywheel, adds only very inefficiently to the rotational inertia of the flywheel. Further, if the hub is stiffer than the composite ring, as the speed of the flywheel increases and the pre-load reduces then the increased mass will lead to stress management problems in the hub.
Yet further, in the existing system, exceeding the maximum stress rating of the composite ring will result in failure. In the flywheel type above, the central support section exerts an outward force on the composite ring due to the pre-load. This force is in the same direction as the centrifugal forces acting on the ring when the flywheel is in rotation. Then, if the stiffness of the hub is lower than the composite ring, the ring must be strong enough to counteract the sum of the preload force and the centrifugal forces when the flywheel is rotating at maximum speed. A further problem with this type of flywheel is therefore that the preload reduces the maximum rotation speed of the flywheel.
A further problem with existing systems is that if a flywheel is to be coupled to, for example, a vehicle transmission, a splined coupling is normally required in order that high transient torque levels (for example when the vehicle gearbox ratio is changed quickly, thus requiring the flywheel to accelerate or decelerate rapidly) may be transmitted to the flywheel without slippage.
A flywheel of the type described in UK patent application 0723996.5, filing date 7 Dec. 2007, overcomes the aforementioned limitations by providing a flywheel having a drive transfer element and a rim comprising a mass element, where the rim and the drive transfer element are coupled by a winding. However, it is desirable with this type of flywheel to have an indication of stress in the flywheel components as the flywheel is rotated at increasing speed.
UK patent application 0902840.8 provides such an aforementioned indication of stress in the flywheel components by incorporating a warning, or indicator, ring into the flywheel. The indicator ring can be mounted to the flywheel with an interference fit, such that residual stresses are set up between the ring and the flywheel. The level of interference fit, or preload, and the relative stiffnesses of the ring and the part of the flywheel onto which the ring is mounted, are chosen such that when the flywheel is rotated at or in excess of a pre-determined trigger speed, the preload is substantially overcome by centrefugal forces, causing the ring and support member to at least partially separate. The ring is then able to move on the flywheel, causing an “out of balance” condition, resulting in a vibration which is detectable as an indication of stress in the flywheel components.
A further problem with existing flywheels is the need to finely balance the rotating mass of the flywheel. Since the kinetic energy stored in a rotating flywheel is proportional to {acute over (ω)}2 (where {acute over (ω)} is the angular velocity of the flywheel), increasing the maximum rotational speed of a flywheel allows more energy to be stored in a flywheel of a given mass, and thereby increases the energy storage density of such a flywheel. However, as the rotational speed increases, the balance of the assembly becomes more critical, as does proving the structural integrity of the flywheel. Furthermore, the cost of balancing a flywheel generally increases with the level of accuracy of balance required.
A further problem when balancing composite flywheels, such as the type described in UK patent applications 0723996.5 and 0902840.8, is that only a limited amount of machining/processing can be performed on the composite component (i.e. the mass bearing rim) without severely affecting the structural integrity of the composite. This thereby affects the simplicity of the balancing process, since material has to be removed from the flywheel at a location away from the composite rim.
A further problem is that existing methods for balancing flywheels generally incorporate machining and/or grinding and/or drilling of material from the flywheel. Not only can (as previously mentioned) such machining and flash or grinding and/or drilling of material from a composite flywheel compromise the structural integrity of the composite part, but furthermore, such machining limits the accuracy of balancing obtainable in at least the following two ways. Firstly, the accuracy of the balancing operation is limited by the trueness of the lathe shaft onto which the flywheel is mounted during the machining operation, and by the accuracy of the mounting of the flywheel mass to the lathe shaft. Secondly, the balancing accuracy is limited by the minimum thickness of material which can be removed in the machining/grinding/drilling process, which in turn may be affected by the skill of the operator and/or (if the machine tool is computer numerically controlled) by the precision of the CNC machine. This is made more acute, since the material removed from the flywheel is necessarily dense (in order to maximise the energy storage density of the flywheel).
It is desirable therefore for a method to be found for simply and quickly balancing such a flywheel to a high degree of accuracy. It is also desirable that the method should simultaneously prove the structural integrity of the flywheel. Such a method would save time, production cost, capital cost, and would also increase the performance and reliability of the flywheel.
Existing flywheels are sometimes constructed such that the rotating mass of the flywheel rotates inside a chamber containing a vacuum. Operating the rotating mass inside a vacuum is advantageous since it reduces energy losses due to air resistance (also known as windage). However, in order to transfer energy into and out of the rotating flywheel mass, a coupling means is required. Some existing flywheels use a rotating shaft passing through a rotating seal in the vacuum chamber to couple torque from an energy source to the flywheel energy storage means. Rotating seals are never perfect, however, since they inevitably leak and therefore require an environmental management system to be coupled to the vacuum chamber in order to maintain the vacuum despite leakage. Furthermore, the seals become more “leaky” with age and as rotational speed increases, and also wear more quickly at higher speeds. The use of rotating seals is therefore undesirable. The mass, volume and cost of such an environmental management system is undesirable.
Magnetic couplings can be used with flywheels to transfer torque through a vacuum chamber wall, thereby obviating the need for rotating seals. However, the torque transmission capability of such magnetic couplings using permanent magnets has previously been found to be lacking in torque transmission capability.
This has been found to be at least partly because the magnetic flux which passes between the poles of the two rotating members, for a given magnetic pole strength, is limited by the “air gap” between the two members. The air gap in fact, comprises the air gap between the outer rotating member and the vacuum wall, the vacuum wall itself, and a vacuum gap between the vacuum wall and the inner rotating member. Since the vacuum chamber wall must be structurally strong enough to support atmospheric pressure, its thickness is necessarily significant, resulting in a large “air gap” between the inner and outer rotating members.
Existing arrangements have sought to overcome this limited torque coupling capability by employing electromagnetic poles in order to increase the magnetic strength and thereby increase torque coupling capability. However, the use of electromagnetic poles requires an energy conversion, thereby reducing the efficiency of the energy storage flywheel (since the electromagnets require electrical power to operate them, which must be sourced from the energy stored in the flywheel). Furthermore, the additional control and power electronics associated with electromagnetic couplings significantly increases the size, and weight of a flywheel energy storage system incorporating such an electromagnetic coupling, thereby further reducing the energy storage density of such a flywheel energy storage system, both in terms of mass and volume. A method of coupling energy into and out of an energy storage flywheel operating in a vacuum chamber, which is efficient in terms of mass, volume and energy is therefore required.
A further problem with existing flywheels is that while the flywheel itself should be able to rotate at a high angular velocity, the drive shaft which invariably couples the flywheel to an energy source or sink (such as an engine or transmission) and associated components which are outside of the vacuum chamber suffer losses associated with air resistance (or “windage”).
Magnetic gears use arrays of magnets (for example, permanent magnets) and stationary pole pieces to transfer torque between rotatable members, for example driveshafts. They exhibit reduced wear when compared to conventional mechanical gears. However, their torque transmission capability is dependent on the rotational position of the magnets with respect to each other, and therefore varies as the shafts rotate. For example, when torque transfer capability is plotted on a graph against angular position, severe peaks and troughs in the torque curve can be exhibited. This is known as “cogging” and leads to a set of undesirable characteristics.
Firstly, peaks and troughs in the torque curve lead to the magnetic gear having a variable “pull-out” torque with meshing position. That is, the torque required before the gears will slip out of mesh varies depending on the rotational meshing position. Therefore, such a gear set for transmitting a given level of torque must be designed such that its minimum torque coupling capability, as represented by one of the troughs (shown at around 20 Nm in FIG. 26) in the torque curve, is greater than the design torque handling figure. For this, the magnet arrays must be sized appropriately larger, and this also normally results in excess torque coupling capability at certain meshing positions, representing an inefficiency. Thus, the magnet arrays are normally sized larger than that which would be necessary if the torque curve more closely followed the mean torque handling capacity, thereby increasing their cost and size, and reducing the energy storage density of a flywheel incorporating such a magnetic gear.
Furthermore, since the angular offset between the input and output shafts of a magnetic gear varies according to the torque applied and to the torque coupling capacity at a given meshing position, if the torque coupling capacity varies with meshing position then this will result in a torsional vibration in the shafts. Such a torsional vibration can reduce the life of the associated mechanical components, and/or can result in failure and/or disengagement. This is an especially serious problem if the rotational speed is such that the frequency of the torsional vibration coincides with a resonance of the mechanical system. It would therefore be advantageous if the variation between the peaks and troughs in the torque curve could be reduced or eliminated. This would allow smaller, cheaper, magnet arrays to be used, since the minimum torque coupling capability would then be much closer to the mean torque coupling capability. Torsional vibration of the shafts would also be reduced, allowing cheaper, lighter and smaller components to be used. A flywheel energy storage system employing such smaller, cheaper and lighter components would have a higher energy storage density.